Interface Reconstruction from Ruddlesden–Popper Structures Impacts Stability in Lead Halide Perovskite Solar Cells

Abstract The impact of the bulky‐cation‐modified interfaces on halide perovskite solar cell stability is underexplored. In this work, the thermal instability of the bulky‐cation interface layers used in the state‐of‐the‐art solar cells is demonstrated. X‐ray photoelectron spectroscopy and synchrotron‐based grazing‐incidence X‐ray scattering measurements reveal significant changes in the chemical composition and structure at the surface of these films that occur under thermal stress. The changes impact charge‐carrier dynamics and device operation, as shown in transient photoluminescence, excitation correlation spectroscopy, and solar cells. The type of cation used for surface treatment affects the extent of these changes, where long carbon chains provide more stable interfaces. These results highlight that prolonged annealing of the treated interfaces is critical to enable reliable reporting of performances and to drive the selection of different bulky cations.